In a pressurized water nuclear reactor, the hot water passing out of the reactor (primary circuit water) is fed into a heat exchanger, the steam generator, where it transfers its heat to another water circuit (secondary circuit) without any direct contact with the water of the reactor. The secondary circuit water vaporizes during the heat exchange and supplies a turbine.
Such a steam generator is generally constituted by an outer, pressure-resistant envelope of considerable height, within which is placed a secondary (vaporization) enclosure covering a bundle of tubes traversed by the pressurized primary water. The secondary circuit is supplied with water in the steam generator, normally in the upper part of the latter. This feed water flows towards the bottom of the steam generator, outside the vaporization enclosure, before entering the latter by its lower part so as to come into contact with the tube bundle and be vaporized.
A mixture of water and steam, which becomes ever richer in steam, flows within the vaporization enclosure in the upward direction and passes out through the upper part of the enclosure into the generator steam dome, which is located between the upper part of the vaporization enclosure and the upper part of the outer envelope.
The steam dome is provided with devices for separating the water and the steam, enabling the steam to be dried before it passes out of the generator. These devices normally comprise a first stage located at the outlet of the vaporization enclosure and constituted by cyclone separators where a large part of the water is eliminated by centrifuging. It also comprises a second stage positioned above the first and formed e.g. by herringbone plates.
The cyclone separators are spiral deflector cylindrical tubes fixed to the ends of tubular columns, which are in turn fixed to the roof of the vaporization enclosure and which communicate with the latter. The mixture of water and steam from the vaporization enclosure thus transits the tubular columns before undergoing a first separation in the cyclone separators. In normal operation, the tubular columns are partly immersed in secondary water.
In order to increase the performance characteristics of the separator and better utilize the volume available in the steam dome, it is advantageous to use small diameter cyclone separators. This reduction in the diameter of the cyclones is, however, accompanied by an increase in their number. For example, for a steam generator with a heating power of 1000 MW operating at 75 bars, it is necessary to have 120 to 140 200 mm diameter separators. Moreover for motor load considerations in the recirculation loop or water reserve in the steam generators, it may be necessary to fix the free level of the secondary water in the steam dome at approximately 2 or 3 meters above the bundle of tubes. As the cyclones must be positioned above this level, the tubular columns linking the cyclones with the vaporization enclosure roof must have a considerable height.
Cyclone separators, no matter whether each is formed by a cyclone fixed to the end of a tubular column welded to the roof of the vaporization enclosure or several cyclones fixed to the end of a larger diameter tubular column fixed to the enclosure roof (CF. FR-A-2 480 905), present maintenance problems in as much as such a group of separators does not permit easy access to the different cyclones for repair or inspection purposes.